The NAB tape cartridge employs an endless loop of tape in a rectangular cartridge which contains a rotatable spool wherein the tape is drawn from the inside of the tape coil on the spool and returned to the outside thereof, the withdrawing of the tape causing a rotation of the spool to effect a rewinding of the returning tape on the outside. The tape travel runs along a straight edge of the cartridge which has three notches therein exposing the tape. Two notches are for record and playback wherein the tape is backed up by resilient pads. The third is for the capstan drive of the tape. The structure is similar to the popular 8-track cartridge except that in commercial usage, the program material is short so that a single mono or stereo track is employed, and in the 8-track cartridge, the pressure roller is integrally incorporated. The maximum tape capacity of a cartridge is about 101/2 minutes playing time at 71/2 i.p.s.
The endless loop of tape is made from a length of ordinary cartridge recording tape with its free ends spliced together with splicing tape. The splicing tape adds to the thickness and the stiffness of the recording tape at the site of the splice, and it is very difficult to splice the tape such that a drop-out of program material will not occur at the site of the splice. Because of this drop-out, the area of the splice, in commercial practice, is carefully avoided in recording whatever message may be placed on the tape.
In commercial broadcasting practice, tape cartridges must be bulk-erased prior to recording on them. Continuous erasure as with a separate erase head in the record-playback mechanism is unsatisfactory for a variety of reasons including, among others, the difficulty of fitting in an erase head, given the physical conformation of the cartridge.
The usual method of effecting bulk erasure is manually bringing an electromagnet powered typically by 60 Hz current generating a strong alternating electromagnetic field close to the cartridge for a moment and then withdrawing the magnet from the cartridge, or vice versa, slowly to a substantial distance before turning off the magnetic field. This must be performed carefully in order to avoid noise-creating residual magnetism in the tape. The procedure is time consuming and troublesome in addition to demanding considerable care and practised skill.
The customary arrangement for driving a record or playback unit is to position the tape within the cartridge immediately adjacent a constantly rotating steel capstan and, upon the record or playback signal, swing a relatively soft-surfaced, freely rotating pinch roller through the drive notch in the cartridge case against the tape to pinch it against the capstan. The soft surface effects good driving engagement for the tape without the application thereto of destructive pressure.